1. Field of the Invention
The present invention relates to a holographic interferometer for precisely measuring the surface configurations of optical elements such as lenses and mirrors, in particular, those of various aspherical optical devices.
2. Description of the Prior Art
Various proposals have heretofore been made with respect to a method of precisely measuring the surface configurations of aspherical optical elements, and, in particular, holographic interferometers are well known to those skilled in the art. A typical holographic interferometer utilizes a hologram standard including a hologram pattern formed by the interference between the wave front of a reference beam and the wave front of a beam reflected or transmitted by an aspherical reference surface, or a hologram standard including a so-called "computer hologram". A computer hologram is commonly made by an electron beam drawing method or the like, after obtaining a hologram pattern from the optical design value of an aspherical reference surface through an electronic computer. The beam reflected or transmitted by an aspherical optical element being measured is diffracted by one of these types of hologram standard, and the diffracted beam is made to interfere with a reference beam, thereby obtaining interference fringes. Finally, based on the physical number and the shapes of the thus-obtained interference fringes, precise measurement is made of the error, from the aspherical reference surface of the aspherical optical element being measured.
These holographic interferometers are typically classified into the following types: Twyman-Green type; Mach-Zehnder type; and Fizeau type. A Twyman-Green interferometer is generally arranged such that light rays supplied from a light source (a laser) are split into two light beams by a beam splitter, one of the beams being used as a reference beam while the other is made to pass through the optical element being tested and a hologram standard, so as to obtain light in diffraction. The diffracted beam light is made to interfere with the reference beam. A Mach-Zehnder interferometer commonly has a construction wherein light rays supplied from a light source (a laser) are split into two light beams by a beam splitter, one of the beams being converted into a reference beam by diffraction by a hologram standard while the other is shone onto the optical element being tested, thereby forming an object light beam, and both beams are made to interfere with each other.
A Fizeau interferometer has the construction shown in FIG. 18. As shown, a light beam emanating from a light source (laser) LS is collimated by a collimator lens C and is then reflected from a beam splitter BS which consists of an inclined half mirror disposed between a focusing lens L.sub.1 and a divergent lens L.sub.2. The reflected beam is then made to be incident upon the divergent lens L.sub.2 as an incident light beam l.sub.1. After the incident beam l.sub.1 has been diverged by the divergent lens L.sub.2, the diverged beam is made to be incident upon a spherical reference surface R. The incident beam is partially reflected by the surface R, returns along the same optical path as that of the incident light beam l.sub.1, and passes through the beam splitter BS, a hologram standard H and the focusing lens L.sub.1. Finally, the light beam passes through the opening of a spacial filter SF in the form of a zero-order reference beam.
In the meantime, the incident light beam transmitted through the spherical reference surface R is refleted by an optical element T being tested (or aspherical concave mirror), to obtain an object light beam. The object light beam is then made to travel in the reverse direction and is transmitted through the beam splitter BS. The component of the transmitted light beam which is not diffracted by the hologram standard H, that is, the zero-order light, is cut off by the spacial filter SF. On the other hand, the component of the transmitted light beam which is diffracted by the hologram standard H, for example, the first-order diffracted light beam, is passed through an opening in the spacial filter SF and forms interference fringes on an interference screen or phgotograhic film as it is combined with the zero-order reference beams thereon.
3. Problems to be Solved by the Invention
The above-described dual optical path types of Twyman-Green and Mach-Zehnder interferometers have the following drawbacks. Since it is necessary to use a large number of optical components such as lenses and mirrors for each optical path, the structure becomes inevitably complicated and the manufacturing cost is increased. In addition, any difference or error in the production or the optical arrangement of the individual optical components directly affects the interference fringes which can be observed, thereby lowering the precision of the measurement.
The Fizeau interferometer described previously with reference to FIG. 18 has no drawbacks such as those of the dual optical path type of interferometers. However, the hologram standard H and the collimator lens C require substantially the same diameter, thereby raising a problem in that it is difficult to produce a hologram standard as a computer hologram.
In addition, since the inclined beam splitter BS is disposed within the parallel pencil of rays defined between the focusing lens L.sub.1 and the divergent lens L.sub.2, the diameter of the splitter BS becomes close to 1.5 times as large as that of the collimator lens C, thereby making it difficult to produce this type of interferometer with a high accuracy.
The Fizeau interferometer further involves the following disadavantage. If the optical piece to be tested is shaped in a greatly aspherical form, a high-density hologram pattern (or interference pattern) is formed on the hologram standard H. In this case, if a computer hologram is to be made by an electron beam drawing method, then huge quantities of calculations and drawing data are necessary.
As described above, prior-art interferometers have the disadvantage in that it is impossible to increase the size of each optical component and the density of the hologram pattern on the hologram standard. In consequence, only a test piece with a small diameter and a small degree of asphericality can be measured, owing to the small effective diameter of the conventional interferometer and the low density of the hologram.
While, a measurement method using an interferometer normally includes on-axis and off-axis measurement methods, the on-axis method is a method in which the object light reflected from a test piece is made to be coaxial with respect to the reference light from a reference surface. In this method, since the spacial frequency of the hologram standard used for measurement can be reduced, it is possible to measure a test piece which has a greatly aspherical surface. However, zero order and higher order lights diffracted by the hologram standard are superposed on the optical axis, although their focal lengths differ from one another. Even if a spacial filter is located at the focal position of the first-order diffracted light in order to select that light, the spacial filter allows the passage of part of the zero-order and second-order, and higher-order diffracted light. For this reason, the on-axis method has the disadvantage in that the central portion, including the optical axis, cannot be measured.
On the other hand, unlike the on-axis method, the off-axis method does not have the above-described portion which cannot be measured. However, when compared with the on-axis method, since the spacial frequency of the hologram standard is increased, only test pieces having small asphericality can be measured, because of limitations on alignment and the production of the hologram standard.
Since the off-axis angle depends on the kind and asphericality of each test piece, an optimum off-axis angle is determined before the hologram standard is produced. The holographic interferometer is so constructed that it is incapable of measuring by both the on-axis and off-axis methods, with the off-axis angle being variable.
Furthermore, in conventional interferometers the hologram standard must be precisely located at a predetermined position in order to carry out the measurement precisely. Location error of the hologram standard causes measurement error or lowers the measurement precision.
Conventionally, the adjustment for locating the hologram standard is carried out so that the optical piece to be tested is supported by holders and the interference pattern that appears is reduced the smallest one. However, it takes a long time to perform the above-mentioned adjustment and every time that the optical piece is to be tested, and both it and the hologram standard are exchanged for new ones, it is necessary to perform this adjustment.
4. Summary of the Invention
Accordingly, it is an object of the present invention to provide a holographic interferometer capable of measuring the surface configuration of even a test piece which has a large diameter and a large degree of asphericality.
It is another object of the present invention to provide a holographic interferometer which comprises a small number of components and wherein the individual components, in particular, a beam splitter and a hologram standard, have a low production cost so that high-precision measurements can be performed even with a low-precision hologram standard.
Another object of the present invention is to provide a holographic interferometer that is capable of measuring by both the on-axis and off-axis methods, with the off-axis angle being variable.
It is, furthermore, another object of the present invention to provide a method of setting a measurement hologram standard, and an apparatus for setting a measurement hologram standard that does not require an adjustment to be carried out for locating it every time the test piece and the hologram standard are exchanged for new ones.
According to the invention, there is provided a holographic interferometer comprising: a source of laser beams; a condenser lens for converging a laser beam supplied from said laser-beam source; a pinhole which allows the passage of said converged laser beam; a collimator lens having a focal point which lies on said pinhole; a beam splitter disposed in an inclined manner between said pinhole and said collimator lens; a spacial filter disposed on the optical axis of a light reflected from said beam splitter and at an optically conjugate position relative to said pinhole; and a hologram standard disposed between said spacial filter and said beam splitter.
According to another aspect of the invention there is provided a holographic interferometer comprising: a collimator lens for collimating light from a light source and projecting said collimated light on a piece being tested; a referencebeam producing element disposed on the side of an exit of said collimator lens so as to reflect part of light from said collimator lens, thereby obtaining a reference beam; a beam splitter disposed on the side of an entrance of said reference-beam generating element, said beam splitter being inclined with respect to the direction normal to the optical axis of said collimator lens in order to reflect said reference and object light scattered from said test piece; a hologram standard disposed on the optical axis of light reflected by said beam splitter; and an observation optical system for observing interference fringes between said object light diffracted by said hologram standard and said reference beam, wherein said reference-beam generating optical element can be made to be inclined with respect to the direction normal to said optical axis of said collimator lens, and said observation optical system can be swivelled through a given angle about a substantial center of an exit pupil of said collimator lens.
In still another aspect of the present invention, there is provided a method of setting a measurement hologram standard comprising the steps of: locating an adjustment hologram standard with a first alignment mark at a predetermined position; aligning an index on a reticle of an alignment optical system with said first alignment mark; and locating a measurement hologram standard having a second alignment mark so that said second alignment mark agrees with said index, said second alignment mark being formed at a position geometrically equivalent to that of said first alignment mark.
Another aspect of the present invention provides an apparatus for setting a measurement hologram standard comprising: an adjustment hologram standard having a first alignment mark; holder means for selectively holding an adjustment hologram standard and a measurement hologram standard having a second alignment mark geometrically equivalent to the position of said first alignment mark; alignment means for locating said adjustment hologram standard at a predetermined position by said holder means; and alignment optical systems respectively having a movable reticle which is disposed on said holder means and has an index so formed as to correspond to said first and second alignment marks.
The above and other objects and features of the present invention will become apparent from the following descriptions of the preferred embodiments and referring to the accompanying drawings.